I used to be shocked lately when a (practically) two-decade-old design thought of mine for take-back-half temperature management received a query from reader John Louis Waugaman. John says he wants a solution to management the temperature of batches of rising bread dough. I’m glad he could be contemplating making use of my TBH circuit to his drawback, but it surely actually is form of overkill in that context. So, I started pondering whether or not an easier topology would possibly remedy his dough incubation drawback in addition to the TBH circuit would whereas saving some value and energy. Plus, there’s a backstory.
Wow the engineering world together with your distinctive design: Design Concepts Submission Information
For a protracted whereas I’ve been keen on a very elegant, correct, and (very!) cheap methodology (∆Vbe) for sensing and controlling temperature utilizing odd, uncalibrated bipolar transistors. I first noticed it defined in an utility notice by famed analog guru Jim Williams (see web page 7 for ∆Vbe idea).
I’m at all times expecting alternatives to make use of ∆Vbe and this risk of offering low-cost, correct, and calibration-free temperature management in a cool culinary context was too good to let go by. Determine 1 reveals the brand new circuit I cooked up, (loosely) based mostly on Jim’s recipe.
Determine 1 Delta-Vbe sensor Q1 is programmed through R1 for desired setpoint temperature in Kelvin = R1/100 = 312oOkay = 39oC for R1 = 31.2k. Asterisked Rs needs to be precision sorts (1% or higher).
The R2 R3 D5 D6 community drives Q1 within the magic 10:1 present ratio ∆Vbe measurement cycle described by Williams. Word that absolutely the currents equipped to Q1 aren’t any extra correct than the uncooked unregulated 60-Hz line voltage that creates them, however that doesn’t have an effect on ∆Vbe accuracy. All that counts is their 10:1 ratio which is ready unbiased of line voltage variation solely by the precision of (R2/R3 + 1) = 10.
This makes Q1 generate a PTAT (proportional to absolute temperature) AC sign equal to oOkay/5050 volts peak-to-peak that follows the 120-Hz log(|sine|) waveshape proven in Determine 2.
Determine 2 Q1’s ∆Vbe PTAT log(|sin(r)|) oOkay/5050 waveform. (Yaxis = volts, Xaxis = radians Pink = common worth = AC baseline)
The PTAT sign Vpp is boosted by A1a’s acquire = –2,742,160/R1, then in contrast by A2 to its precision (2.50 V +0.4%) inside shunt reference (thanks once more, Konstantin Kim, for locating the versatile AP4310A!).
A2’s output stays at zero, holding Q2 off, whereas Q1’s temperature and the PTAT sign are beneath setpoint. This enables 120-Hz pulses coupled via C3 to achieve Q3’s gate, swap it on, and apply energy to the heater. When the heater warms Q1 (and presumably the dough) to the programmed temperature, then the PTAT waveform rises above A2’s reference voltage. This makes A2 begin turning Q2 on which diverts the TRIAC gate pulses to floor. That turns Q3 and the heater off permitting Q1 to chill, and many others., and many others. The ensuing biking completes a thermostasis suggestions loop to make the dough develop.
About Q3: Although heater drive is unipolar, I chosen a TRIAC as an alternative of an SCR for Q3. This wasn’t to get bipolar functionality however relatively as a result of the TRIAC has a better max gate present score. This makes Q3 capable of shrug off the 2-A inrush doable at power-up, which could vaporize an SCR gate.
D7 gives a path to floor for C3 return present, stopping it from false triggering Q3.
For heater responsibility, John instructed an incandescent gentle bulb. I agree radiant heating ought to work effectively. Since Q3’s most responsibility issue is 50%, a 100-W bulb can be nearly excellent for a most heating energy of ~60 W. Plus, a bonus good thing about lowered voltage can be a decrease filament temperature. This could make an odd tungsten bulb final many hundreds of hours.
It might be simply what John kneads.
Stephen Woodward’s relationship with EDN’s DI column goes again fairly a great distance. Over 100 submissions have been accepted since his first contribution again in 1974.
Associated Content material
- Temperature controller has “take-back-half” convergence algorithm
- Take-back-half thermostat makes use of ∆Vbe transistor sensor
- Take-Again-Half precision diode cost pump
- 20MHz VFC with take-back-half cost pump
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